Wireless time reference system and method

ABSTRACT

Various methods and apparatuses that utilize a wireless time reference system are provided herein. One example method involves calibrating independent, spatially-located clocks of a geoposition system in order to geolocate an object having an associated object tag. The example method may include transmitting an RF pulse pair, receiving the pulse pair at multiple locations, utilizing respective frequencies of first and second spatially-located clocks to produce count values to effect measurement of an interarrival interval at each of multiple locations, determining a ratio of count values relative to said first and second spatially-located clocks, and utilizing said ratio to calibrate time indications of said clocks. Other related methods and apparatus are also provided.

CROSS-REFERENCE TO RELATED PATENTS AND PATENT APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12,379,189, filed on Jun. 15, 2009, entitled “Wireless TimeReference System and Method”; which is a divisional application of U.S.application Ser. No. 11/480,982, filed on Jul. 6, 2006, entitled“Wireless Time Reference System and Method” (now U.S. Pat. No.7,492,316); which claims priority to U.S. Provisional Application Ser.No. 60/752,950, filed on Dec. 23, 2005, entitled “Wireless ObjectLocation System and Method”; all of which are hereby incorporated byreference in their entirety.

BACKGROUND

This invention concerns timing synchronization or determination of timecorrections to be applied to timing devices of independent RF receiversgenerally but is disclosed herein in connection with implementation in awireless object location system and method.

In U.S. Pat. No. 6,882,315 (“the '325 patent”), a precision objectlocation system is described in which cycling of local clocks ofmultiple receivers was frequency-locked (but not necessarilyphase-locked) with a common clock frequency (e.g., where a common clocksignal is conveyed over low-cost CATS cables from a centralprocessor/hub to each of the multiple receivers). Such a system may usean active (i.e., transmitting) reference tag to enable determination ofor to compensate for relative phase offsets between individual clocks ofthe receivers. With the use of ultra wideband (UWB) or short pulsewaveforms as timing signals, location accuracies and precisions withinone foot were achieved. Synchronizing local receiver clocks is extremelyimportant to obtaining precise positioning accuracy and much effort inthe prior art has been directed to aligning timing references in each ofmultiple receivers or monitoring stations in a geopositioning or anobject location system. Thus, shielded or unshielded (twisted wire pair)cabling was used to interlink the receivers via a common timingreference. Alternatively, where no wireline link is provided, prioruntethered (i.e., wireless) systems utilized extremely accurate, albeitexpensive, local timing references at each receiver but even then thetiming accuracy of internal clock circuits and concomitant positioningaccuracy is still subject to temperature changes, frequency drift orclock skew (which necessitated periodic synchronizing with a commonsource).

In many applications, particularly those outdoors or in areas in whichconventional wiring is either not possible (e.g., on a large cattleranch for tracking livestock) or exorbitantly expensive (e.g., within anoil refinery for tracking safety personnel), it is desirable toeliminate wire lines running from a central processor hub to individualreceivers or monitoring stations. In furtherance of such goal, thepresent invention proposes an alternative system and method to obtainsynchronization or offset information for the individual receivers.

In Anderson et al. (U.S. Pat. No. 5,469,409) (“the '409 patent”), amethod is described to wirelessly phase-lock individual receivers thatoperate with independent internal local clocks having no common orexternal timing reference. Here, a reference tag is used to performsynchronization. In operation, a transmission from the reference tag isreceived by multiple independent receivers. Each receiver, knowing itsown location and the exact position of the reference tag, may thencalibrate its own clock by calculating/measuring the precise propagationtime for the tag signal to reach the receiver. In addition, knowing theexact cable delay from each receiver's antenna/preamplifier (AP) node toa collector (C) node, a processor hub may also compensate for therelative timing offsets between the individual receivers. Thiscomputation is accomplished by subtracting the sum of the propagationtime and cable delay from the measured arrival time of the reference tagtransmission at each receiver (as measured in a local time coordinatesystem at the receiver). The resultant estimates (one for each receiver)of the epoch time of the reference tag transmission are then suitablyadjusted and aligned by the central processor so as to provide a commontime reference point for subsequent transmissions from other(non-reference or object) tags.

A disadvantage of Anderson's technique is that, if relativelyinexpensive internal clocks are used as suggested, updates from thereference tag must be received at a sufficiently fast rate or clockdrift between receivers (e.g., due to a simple frequency offset) willcreate significant location errors. In essence, Anderson “pins” theepoch time of a tag transmission event for all receivers after a singlecalibration, but does not compensate for time-of-arrival drift due toclock frequency offsets. For example, in an Anderson implementation, ifone receiver's clock frequency differs from that of another receiver by20 parts per million (ppm), a one nanosecond difference in reference tagtimes-of-arrival is accrued in 500 microseconds. Thus, a calibrationcycle must occur every five milliseconds (or 200 times per second) tomaintain an accuracy of ten nanoseconds, or approximately ten footresolution (based on distance of RF signal propagation during tennanoseconds).

The present disclosure describes a system and method for significantlyimproving performance over prior systems and methods, such as thatcontemplated by Anderson et al., while fully enabling a wirelessimplementation of methods and systems described in commonly-owned '315patent without sacrificing positioning accuracy. The present inventionadditionally allows further improvements and advantages over the methodand system described in the '325 patent.

BRIEF SUMMARY

According to a first aspect of the invention, a system providesnormalization of time (e.g., synchronizing or calibrating clocks, orproviding an indication of skew compensation) of local clock circuits ofplural spatially-located monitoring stations. One embodiment comprises areference transmitter to transmit at least two RF pulses (UWB) thatdefine a time reference interval wherein local clock circuits haverespective detection circuits that detect and measure time durations ofthe time reference interval at respective receivers of the monitoringstations, and a processing device that determines a relationship (e.g.,ratio) between time durations measured at receivers of at least twomonitoring stations and that effects normalization of time (e.g.,calibration) of local clock circuits according to the relationship. Oncenormalized, the local clock circuits may accurately measuretimes-of-arrival of subsequent object tag transmissions in an objectlocation system. The processing device may effect normalization oftime-of-arrival measurements among receivers of the monitoring stationsby controlling the frequency of at least one local clock circuit of amonitoring station to maintain the relationship fixed (e.g., one-to-one)with respect to a frequency of a local clock of a selected other (i.e.,master) monitoring station.

In a more specific embodiment, each of the local clock circuitscomprises a ring counter that detects and latches a clock count tomeasure the duration of the time reference interval and/or theprocessing device (i) wirelessly receives at a central processing hubdigital information representing clock count information of themonitoring stations and time-of-arrival measurements of an object tagtransmission and (ii) effects normalization of the time-of-arrivalmeasurements according to a ratio of clock count information.

In another embodiment, a method of providing normalization of time of(e.g., synchronizing, calibrating, or providing skew compensation for)local clock circuits of two or more spatially-located receivers ofrespective monitoring stations comprising transmitting at least two RFpulses to define a time reference interval; detecting RF pulses at thereceivers; at respective monitoring associated with the receivers,measuring durations of the time reference interval according to thedetecting step; determining a relationship (e.g., ratio) betweendurations measured at the monitoring stations; and providingnormalization of time of the local clock circuits at two or moremonitoring stations according to the relationship. The method mayfurther include counting unit increments of time (e.g., using a ringcounter) to measure the time reference interval between the UWB pulsesand then latching a clock count to measure the durations; and theprocessing step may include normalizing time-of-arrival measurementsamong receivers of the monitoring stations by controlling the frequencyof at least one local clock circuit of a monitoring station to maintainthe relationship fixed (e.g., one-to-one) with respect to a frequency ofa local clock of a selected other (i.e., master) monitoring station. Ina wireless scheme, the method may further include wirelessly receivingat a central processing hub digital information representing clock countinformation and time-of-arrival measurements of an object tagtransmission; and then normalizing the time-of-arrival measurementsaccording to the relationship.

In yet another embodiment, there is provided a time reference system forindependent clocks of spatially-located wireless receivers of an objectlocating system that determines the position of an object. Thisembodiment comprises a reference tag transmitter to transmit a pair ofshort-pulse signals that define a time reference interval therebetween,a first wireless receiver that receives the short-pulse signals togenerate a first count value according to a first local clock indicativeof a time interval between pulses of the pair of short-pulse signals, asecond wireless receiver that receives the short-pulse signals togenerate a second count value according to a second local clockindicative of a time interval between pulses of the short-pulse signals,and a processor responsive to the first and second count values of thefirst and second receivers to determine an offset between the first andsecond local clocks whereby to enable determination of object locationaccording to the offset. In a preferred embodiment, the first and secondreceivers generate digital representations of the count values and theprocessor hub wirelessly receives the digital representations of thefirst and second count values.

In yet another embodiment, a time reference system for independentclocks of spatially-located wireless receivers of an object locatingsystem that determines the position of an object comprises a referencetag transmitter to transmit a pair of short-pulse signals that define atime reference interval therebetween, a first wireless receiver toreceive the short pulse signals to generate a first count valueaccording to a first local clock in order to measure a time intervalbetween pulses of the pair of short-pulse signals, a second wirelessreceiver to receive the short-pulse signals to generate a second countvalue according to a second local clock in order to measure a timeinterval between pulses of the short-pulse signals, and a processorresponsive to the first and second count values of the first and secondreceivers to determine an offset and to use the offset to effectadjustment of the frequency of at least one of the first and secondlocal clocks to maintain relative synchronization thereof.

In yet further embodiment, a method of normalizing independent clocks ofrespective receivers of remote monitoring stations comprisestransmitting an ultra-wideband (UWB) pulse pair, determining at firstand second monitoring stations a respective clock count indicative of alocally measured time interval between the pulse pair, each the clockcount being derived by incremental measurement of time units (e.g., by alocal clock/counter/oscillator/multivibrator/delay-line/incremental timeindexer/time unit measurement circuit associated with respectivereceivers), determining a ratio (e.g., by using programmed processormodule) between clock counts of the first and second monitoringstations, and utilizing the ratio as a reference to normalize time oflocal clocks that generate each the clock count. The utilizing step mayfurther include normalizing time by synchronizing local clocks thatgenerate each the clock count or by applying a correction totime-of-arrival measurements taken at respective monitoring stations.

A further embodiment of the invention comprises a method of normalizingtiming references of spatially-located receivers in a geoposition systemcomprising utilizing a UWB pulse pair transmission to determine offsetsbetween clocks of the receivers.

A further embodiment of the invention comprises a method calibratingindependent, spatially-located clocks of a geoposition system in orderto geolocate an object having an associated object tag. The methodcomprises transmitting an RF pulse pair, receiving the pulse pair atmultiple locations, utilizing respective frequencies of first and secondspatially-located clocks to produce count values to effect measurementof an interarrival interval at each of multiple locations, determining aratio of count values relative to the first and second spatially-locatedclocks, and utilizing the ratio to calibrate time indications of theclocks. This may further include utilizing a ratio of an initial pulsepair to maintain synchronization of the clocks during subsequent pulsepair transmissions; or wirelessly transmitting the count values to acentral processing hub that determines object position according to theratio.

In yet an additional embodiment of the invention, an object locationsystem to locate an object equipped with an object tag transmittercomprises a reference transmitter to transmit at least two UWB pulsescomprising short bursts of RF energy, multiple monitoring stationspositionable at known locations, each including a UWB receiver and alocal clock that responds to receipt of the UWB pulses to determine aclock count based on an interarrival interval, each monitoring stationfurther including transmitter to transmit (via cable, wire, over-the-airtransmission) a digital representation of the clock count to a centralprocessing hub, and the central processing hub including a processor tocompute a ratio of count values relative to first and second ones of themultiple monitoring stations and to utilize the ratio to alignindications of timing references of the monitoring stations in order todetermine the position of the object. Further, the processor may computea second count value ratio between first and third ones of themonitoring stations and utilizes the first and second count values toalign respective local clocks to determine the position of the object;the processor may utilize a clock count value of one of the monitoringstations as a common reference in order to compute ratios with respectto clock count values of other monitoring stations; the processor maydetermine object location by aligning local clocks of other monitoringstations with the first monitoring station; or the reference transmittermay transmit multiple pulse pair groups and the processor correctsindications of clock drifts among the monitoring stations in order tomaintain synchronization of local clocks. The digital representation mayfurther include a station identifier to identify respective ones of themonitoring stations and the processing hub may be co-located with one ofthe monitoring stations.

Other aspects of the invention will become apparent upon review of thefollowing description and drawings. The invention, though, is pointedout with particularity by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrate a single-pulse time synchronizing technique used in aprior art system.

FIG. 2 shows a pulse pair reference transmission system uses in oneembodiment of the present invention to determine a timing offset betweenremotely located monitoring stations.

FIG. 3 shows multiple pulse pair groups used for maintaining alignmentof remote receiver clocks on an on-going basis.

FIG. 4 illustrates a system implementation of the wireless objectlocation technique described in connection with FIG. 2 in accordancewith an aspect of the present invention.

FIG. 5 is a flow chart of a method aspect of the present invention.

FIG. 6 depicts a broader aspect of the method contemplated by thepresent invention.

FIG. 7 illustrates a simplified block diagram of a monitoring station.

FIG. 8 depicts a measurement apparatus within a monitoring station ofFIG. 7.

FIG. 9 illustrates waveform diagrams for delay line and latchoperations.

FIGS. 10A and 10B illustrate phase offset between signal counters ofrespective monitoring stations.

DETAILED DESCRIPTION

To help place the invention in perspective, Anderson '409 describes a“zeroth order” technique in which times-of-arrival of a reference tagsignal (as observed at spatially: located receivers) are preciselyaligned using a priori knowledge of the reference tag and receiverpositions (hence, a priori knowledge of reference tag propagation timesfrom the reference tag transmitter to the individual receivers) and APnode to C node cable delays. However, as indicated above, even a smallfrequency offset between receiver timing circuits causes thiscalibration to be quickly lost.

Instead of determining timing alignment of remote clocks in multiplereceivers based on time-of-flight measurement of a reference pulse and apriori knowledge of positions of the reference tag and the receivers,the present invention utilizes a relatively fixed time referenceinterval and a constant or known frequency offset between respectivelocal clocks and/or higher order differences (e.g., frequency drifts,etc.) between or among transmitter and receiver circuits of independentmonitoring stations. The offset at each monitoring station is determinedby measuring a time reference interval established by a pair of RFpulses, e.g., a pulse pair, of UWB pulses. In one embodiment, each UWBpulse comprises a short burst of RF energy have a few cycles. Since thetime reference interval is relatively constant for a given transmissionmedium in a region of the monitoring stations (i.e., the signalpropagation speed throughout the medium is substantially constant), thetime interval between pulses of the pulse pair provides a uniform timereference or metric for the timing circuits of the spatially-locatedmonitoring stations. In a wireless object location system employing UWBpulse transmissions, this allows for use of less expensive timingcircuits for the remote clocks while maintaining sub-foot resolution inpositioning accuracy.

FIG. 1 illustrates RF timing pulses used in a prior art technique, asimplemented by Anderson '409, where a single reference tag pulsetransmission 10 initiates a calibration cycle to compensate for timingoffsets between two (or more) receivers as reflected by the staggeredtimes of receipt of pulses 12 and 14 at receivers 1 and 2 (not shown),respectively. This process is repeated continuously at a rate sufficientto maintain desired relative clock accuracy as indicated above, i.e., toachieve a predetermined minimal acceptable difference between expectedand actual times of receipt of pulses 12 and 14 according to knownlocations of the reference tag transmitter and the receivers. Valuesτ_(p1) and τ_(p2) are a priori known propagation times of tagtransmission pulse 10 from the reference tag transmitter to receivers 1and 2, respectively. Values τ_(c1) and τ_(c2) are a priori known (i.e.,measured) cable delays for receivers 1 and 2, respectively. Theexpression (x); represents the value x as measured in a time coordinatesystem of receiver i. Thus, (τ_(p1)+τ_(c1))₁ is the sum of thepropagation time and cable delay for receiver 1 as measured in receiver1's time coordinates.

FIG. 2 illustrates timing signals utilized in a first embodiment of thepresent invention where two reference tag transmission events 20, 22occur. In the illustrated embodiment, events 20, 22 are very shortradio-frequency (RF) or UWB pulse transmissions emitted from a remotereference tag transmitter (not shown). The interarrival (orintertransmission) time 24 (i.e., time reference interval) of pulse pair20, 22 between these events is denoted as τ_(tag) which, in general, maybe an imprecise and potentially time-varying quantity (which depends onthe design and stability of internal clock circuitry driving thetransmissions from the reference tag itself). Each receiver of themonitoring stations, however, receives the same reference tagtransmissions (receiver 1 detects the two reference tag transmissions20, 22 as pulse pair 26, 28 while receiver 2 detects the reference tagtransmissions as pulse pair 30, 32) and, by performing a measurement ofthe interarrival time in their respective time coordinate referencesystems, obtains an estimate of τ_(tag). As shown in FIG. 2, receiver 1generates an estimate of interarrival time τ_(tag) as (τ_(tag))₁ andreceiver 2 generates an estimate of interarrival time tag as (τ_(tag))₂.The ratio of these measurements, (τ_(tag))₂/(τ_(tag))₁, provides a verygood estimate of the ratio of (or skew between) the respective clockfrequencies of individual receivers 1 and 2. Based on the ratio, timingdifferences are readily ascertainable using techniques known in the artwhen computing the location of an object tag. Computation techniques, inan illustrated deployment of the present invention, are moreparticularly described in the commonly-owned '315 Richley et al. patent.

As an example, let τ_(tag) have a nominal value of one second, andsuppose that in one measurement interval τ_(tag) has an actual value of1.000523 seconds. Further, suppose that receiver 1 and receiver 2 haveknown internal clock frequencies of 99.99356 MHz and 100.00118 MHz,respectively. Receiver 1 measures τ_(tag) as (1.000523)(99.99356MHz)=100045857clock counts, and receiver 2 measures τ_(tag) as(1.00523)(100.00118 MHz)=100053481 clock counts. Clock counts may bemeasured by methods and circuit known in the art, or by methods andcircuits described in the '315 patent, such as free-running counters,multi-vibrators, digital counters, tapped delay lines, and/or othercircuits. The ratio of these two count values, or100053481/100045857=1.0000762=8, is an extremely close estimate (for ahigh frequency clock) of the ratio of the two individual clockfrequencies. Note also that, if the next interarrival time between asubsequent pulse pair transmission from the reference tag is 1.000289seconds (e.g., due to frequency drift of the reference tag transmitterclock), the new counts are now 100022458 for receiver land 100030080 forreceiver 2, but the ratio remains constant, i.e.,100030080/100022458=1.0000762 as before. Thus, the actual time spacingbetween pulse pair transmissions of the reference tag becomes irrelevantto achieving positioning accuracy, as is its precise position in space(as required by the Anderson '409 patent).

To describe an exemplary embodiment to determine the ratio, a localprocessor at each monitoring station captures, digitizes, and wirelesslytransmits clock count information to a remote processing hub thereby toenable mathematical determination of correct times of arrival ofsubsequent object tag signals at the processing hub. Alternatively, themonitoring stations themselves may broadcast their clock countinformation with an ID message so that the other monitoring stationsreceive these clock counts and locally determine a ratio relative to aselected one of the monitoring stations, i.e., a master. Thus, the ratiomay be locally used to perform time correction or alignment locally. Inan exemplary implementation, the frequency of the local clock at eachmonitoring station may be varied (e.g., using a voltage controlledoscillator) in order to maintain a one-to-one clock count ratio with theclock count of a master monitoring station. This way, all clocks arecoherent in frequency and no further time correction would be required(except for internal cable or other delays) to measure the time ofarrival of a subsequent object tag transmission.

With knowledge of the ratio δ for any initial or subsequent stabilizedreference tag transmission, the frequency (f₂) of receiver clock 2 canthen be corrected to be equal to the frequency (f₁) of receiver clock 1since f₂˜f₁·δ for any or all subsequent measurements. In general, forany number N of untethered receivers, each receiver clock can beprecisely referenced to that of one of the other receivers. With equal,or near equal, local clock frequencies among the individual receivers,the wireline approach described in the U.S. Pat. No. 6,882,315 patentcan now be followed to attain a remote wireless object location systemor method for the precision location of arbitrary object tags.

Advantageously, a system or method according to the present invention isunaffected by changes (drifts) in inter-transmission time of thereference tag pulse pair. Instead of transmitting a pair of pulses (ormultiple pairs) to define a time reference interval, a series of threeor more pulses may be transmitted and intervals between any one or morepulses may be used as a timing reference interval. Also, the timingreference interval may comprise an average of multiple intervals thatlie between multiple pulses. Further, no knowledge of the reference tagposition is required in a system or method of the present inventionsince the computational results are unaffected by tag position andprimarily depend on the duration of the reference interval betweenpulses.

For better accuracy, the reference tag interval and nominal receiverclock frequency should be large enough so that the product of these twovalues produces a count value with sufficient gradations to yield a goodestimate when taking the ratios of these values. On the other hand, thereference tag interval between pulse pairs must also be short enough sothat the assumption of a constant frequency offset is satisfied. Forexample, in the case where temperature fluctuations occur, theassumption of a constant frequency offset may not be satisfied over verylong interarrival times.

In practice, it has been found that a one-second update rate is adequateto provide a wireless remote object location system or method having thesame or similar accuracy and resolution as the wired counterpartdescribed in the U.S. Pat. No. 6,882,315 patent, i.e. one foot or less.Note that this update rate is many orders of magnitude lower than whatwould be required for a comparable positioning accuracy using theapproach described in the Anderson '409 patent.

The embodiment described above can be further generalized as shown inFIG. 3. Here, multiple pulse pairs 41-42, 43-44 having reference taginterarrival times τ_(tag,i), for i=1, 2, . . . are measured withcorresponding estimates of received pulse pairs 51-52, 61-62, 53-54,63-64 (not all pulse pairs shown) from each j^(th) receiver 1, 2, . . .{τ_(tag,i)}_(j), j=1, 2, . . . . If the computed ratio δ changes from anm^(th) interval to a next interval, the invention advantageously enablescorrection of clock frequencies not only for constant offsets, but alsofor constant accelerations (i.e., frequency drift) and/or higher ordereffects.

The above techniques can be used to remotely and wirelessly adjust eachreceiver clock to the same frequency, with subsequent phase locking ofthe cycling thereof then performed using reference tag information asdescribed in the U.S. Pat. No. 6,882,315 patent. Furthermore, thedigital data stream that is sent from each receiver to the hub/processornode in the U.S. Pat. No. 6,882,315 patent can now be wirelesslytransmitted as well to the hub/processor by using standard,off-the-shelf, wireless technologies such as ISM-band, 802.11a/b/g, orother wireless technique.

FIG. 4 shows one of many implementations of the invention, such as in awireless object location system 70 in which a processing hub 72 includesan antenna 73 to convey digital data to or from a number of receivers ormonitoring stations 74, 76, and 78 via wireless links 73 a, 73 b, and 73c. Alternatively, as indicated herein, any one or each of the monitoringstations may include the functionality of processing hub to effectalignment of the clock frequencies, locally. Transceivers of monitoringstations 74, 76, and 78 link with processor hub 72 through wirelesstransmissions with respective antennas 75, 77, and 79. A local processorand/or timing circuit within the monitoring stations generate countvalue information based on the duration of an interarrival intervaldetected in a pulse pair transmission from reference tag 80, preferablya UWB pulse pair transmission from antenna 81. The UWB pulse pairtransmission travels along paths 81 a, 81 b, and 81 c to the antennas75, 77, and 79 of respective monitoring stations 74, 76, and 78. Asindicated above, processor hub 72 may obtain the count value informationvia a conventional wireless transmission protocol and such informationmay be conveyed after a polling operation performed or initiated by hub72, or alternatively, the monitoring stations may transmit their countvalues sua sponte either periodically or at other predefined intervals.In another embodiment, processor hub may be hardwired with any one ormore of the monitoring stations as described in the '315 patent, ifdesired or practicable, but still implement the synchronization or timecorrection methods and systems described herein. Wireless transmissionsfrom the monitoring stations may include station ID or other informationas described in incorporated '315 patent according to any air interfaceprotocols (e.g., IEEE wireless transmission protocols). Optionally, hub72 may be a stand-alone unit or colocated with any of the monitoringstations. Moreover, the reference tag transmitter 80 may also becolocated with the processor hub 72 or any one of the monitoringstations.

In order to maintain synchronization, reference tag 80 may transmit UWBpulse pairs every second, more or less, or at any other interval(periodic or non-periodic). With accurate offset information on hand inthe form of count values, e.g., count values may be stored in a memory,processor hub 72 computes alignment variances between and among localclocks (e.g., ring counters) of the monitoring stations so that accuratetime-of-arrival data may properly be determined or calculated upondetecting subsequent UWB transmissions from object tag 82. Paths 82 a,82 b, and 82 c respectively convey UWB transmissions from the object tag82 to the monitoring stations 74, 76, and 78. Such UWB transmissions mayalso include ID or other information, as described in incorporated '315patent, which enables multiple objects to be geolocated simultaneously.Instead of UWB transmissions, the invention may employ short or widebandRF pulses.

FIG. 5 illustrates a method according to an aspect of the inventionwhere, at step 90, an RF pulse pair is transmitted to define anintertransmission interval, and correspondingly, an interarrivalinterval when later received at a monitoring. The pulse pair maycomprise two UWB or wideband pulse transmissions. Step 91 includesmeasuring the interarrival interval associated with the pulse pairtransmission at two or more receiving or monitoring stations. In oneembodiment, the interarrival interval may be defined by and measured atthe rising edge of each of the initial and following pulses of the pulsepair transmission but this interval may also be defined by and measuredat other points on the pulse waveform. Measuring may be performed byconventional techniques known in the art, including deploying a ringcounter to count equal increments of time during the interval, and thenlatching a count value at the end of the interval. After measuring theinterarrival interval, a ratio of count values between at least twomonitoring stations is computed at Step 92. This ratio indicates theratio of local clock frequencies of two monitoring stations and thusindicates misalignment of the clocks for subsequently determining, atstep 93, a corrected time-of-arrival of a subsequently transmitted pulsereceived at the monitoring stations. The ratio is preferably determinedat a central processing hub but may be determined locally at any one orall monitoring stations if count value information is shared, i.e.,broadcasted among the monitoring stations. In addition, one monitoringstation may serve as a common master reference for all other monitoringstations to align local clocks of all monitoring stations.

At step 94, time-of-arrival of a subsequent pulse is measured, e.g., bylatching the count status or count value of a cyclic counter asdescribed in the incorporated '315 patent. The latched value, at step95, may then be transmitted to a central processor hub in order todetermine, with the use of counter skew or count value information, thelocation of the object that transmitted a subsequent object tag pulse.

FIG. 6 depicts a broader aspect of the method contemplated by thepresent invention where the method simply comprises of transmitting 96at least two RF pulses to define a time reference interval, measuring 97the time reference interval at two or more monitoring stations toascertain a relationship therebetween (e.g., a ratio or othermathematical relationship), and utilizing 98 the relationship todetermine a time correction in computing object location or aligninglocal clock frequencies at two or more monitoring stations. Timecorrection is preferably performed in software by adjusting the actualtime-of-flight measurements of object tag transmissions according to therelationship. Alternatively, the relationship (e.g., ratio) may be usedto align local clocks, e.g., to alter and/or control local clockfrequency of one or more local clocks to achieve and maintain aone-to-one or other fixed relationship between two or more local clockfrequencies.

FIG. 7 shows a block diagram of an exemplary monitoring station 74 (FIG.4) which includes a measurement apparatus 204 at each of the monitoringstations. Since computations are performed or measurements are taken ateach monitoring, digital, rather than analog, signals may be transmittedto processing hub 72 (FIG. 4). Thus, cable dispersion, which may degradethe integrity of analog waveforms, is avoided.

In a preferred embodiment, Ultra Wideband (UWB) radio data packets 200are transmitted to the monitoring station 201 and intercepted by UWBantenna 218. A UWB transceiver 202 is provided at each monitoringstation. The transceiver can, for example, be designed in accordancewith the system described in commonly-owned, incorporated U.S. Pat. No.5,901,172.

UWB transceiver 202 produces a digital bit stream that is processed bypacket decoding logic 203, which performs packet framing and bit timingas part of an isochronous communication system. In an isochronoussystem, the communication signals carry timing information embedded aspart of the signal. Upon receiving a complete UWB data packet, packetdecoding logic 203 generates and sends an interrupt signal on line 212to the digital signal processor (DSP) 206. Tag ID and a sequential burstcount 210 are also extracted from the packet, and are sent to the DSPfor further processing. Packet decoding logic 203 also generates a TOAtiming pulse 211 that is precisely referenced in time relative to thebeginning or end of a UWB data packet synchronization preamble. Thesynchronization preamble may comprise a few bits of data having a uniquepattern at the beginning of the UWB packet burst so that the UWBtransceiver 202 may determine the validity of the received packet aswell as bit time alignment. The TOA timing pulse is subsequentlymeasured by measurement apparatus 204, which functions as atime-to-digital converter. An output TOA measurement 215 is a digitalresult that is determined in response receipt of the TOA timing pulse.

Upon receiving an interrupt signal, DSP 206 reads the TOA measurement215 along with the optional tag ID and sequential burst count 210, andstores the combined information in the TOA measurement memory 207. Anyadditional information decoded by the packet decoding logic 203 (e.g.,personnel data, cargo manifest, etc.) can also be stored in memory atthis time. In a preferred embodiment, the TOA measurement memory 207operates as a First-In First-Out (FIFO) buffer. Also, in the preferredembodiment, a program (which is typically stored in a FLASH memory, notshown) manages a portion of a general-purpose static RAM to function asthe TOA measurement memory FIFO.

Because packet data and measurement results can be transferred at highspeeds to TOA measurement memory, the monitoring station 201 can receiveand process tag (and corresponding object) locating signals on a nearlycontinuous basis. That is, multiple UWB data packets can be processed inclose succession thereby allowing the use of hundreds to thousands oftag transmitters. Data stored in TOA measurement memory 207 iswirelessly transmitted to the processing hub 72 (FIG. 2) via a wirelessnetwork interface 208 in response to a specific request from theprocessing hub 72. Thus a low-cost, high-latency data network can beused while retaining the ability to continuously receive tag locatingsignals.

In addition, wireless interface 208 has is bi-directional. Interface 208may convey command signals from the processing hub to, for example,instruct DSP 206 to transfer the contents of the TOA measurement memory207 to the processing hub 72. Additional commands include those toadjust UWB transceiver operating characteristics such as gain anddetection thresholds.

Within the monitoring station 201, a timing reference clock signal online 213 is frequency-multiplied using well-known techniques byphased-lock loop (PLL) clock multiplier 205 (e.g., Pericom PI6C918AW),thereby producing a local timing reference signal on line 214. In oneembodiment, the timing reference clock signal on line 213 may have alocal clock frequency of 10 MHz, and the local timing reference signalon line 214 is generated at 100 MHz (i.e., a 10× digital multiplicationfactor). Alternatively, a 100 MHz crystal oscillator may be used toproduce a local clock signal.

FIG. 8 is a block diagram of an example embodiment of the measurementapparatus 204 (FIG. 7). The TOA timing pulse 306 generated by the packetdecoding logic 203 (FIG. 7) is coupled to the input of delay line 302 aswell as to the enable input of digital latches 304 and 305. This timingpulse is indicative of the total propagation delay between the tagtransmitter and the UWB receiver antenna 218.

The timing pulse is asynchronous with respect to the local timingreference signal. Therefore, following the assertion of the timingpulse, the next rising edge of the local timing reference signal causesthe latches to capture the instantaneous outputs of delay line 302 andoutput of digital counter 303.

In the example of FIG. 8, counter 303 runs at a clock frequency of 100MHz. Thus, the value of latch 305 has a ten-nanosecond timingresolution. This is too coarse to be used in most applications, thus, anadditional delay line 302 is also provided. Delay line 302 allowsmeasurement of fine-resolution timing differences between the TOA timingpulse and the local timing reference signal. In this same example, delayline 302 has a one-nanosecond delay per tap. Delay taps of less than onenanosecond separation can be used to achieve even finer rangeresolution. Waveforms associated with delay line 302 are shown in FIG.9, which illustrates waveforms corresponding to the D inputs and Qoutputs of latch 304 when the timing pulse arrives three nanoseconds inadvance of the rising edge of the local timing reference signal.

FIGS. 10A and 10B illustrate a phase offset compensation technique. Ingeneral, there are two forms of synchronization—frequency and phase. Ina frequency-synchronized or locked system, such as illustrated herein,there is no relative drift between time measurement apparatus, althoughthere may be a relative fixed phase offset. In other words, a pluralityof frequency-locked counters (each located within a separate measurementapparatus 204 (FIG. 7)) preferably count at the same rate. However, thecounters are not necessarily aligned to the same count value at anygiven instant in time. This difference is referred to as relative phaseoffset and is illustrated by the graph of count values versus time inFIG. 10B. Note that digital counters #1 and #2 associated with therespective waveforms 256 and 257 have different count values at anygiven point in time.

The problem of phase alignment is typically solved through the use of acounter reset or phase synchronization signal. In contrast, the presentinvention may utilize non-resettable counters. During operation, thecounters have random, but constant, phase offsets. To compensate forrelative phase offsets, a reference transmitter is positioned at knowncoordinates. This transmitter transmits a unique ID code to allow theabove-described processing algorithm to identify latched count valuescorresponding to signals received from this particular transmitter.Since the locations of the transmitter and receivers are all known, theexact phase offsets between counters can be readily deduced as describedabove.

The illustrated embodiments disclosed herein set forth variousapproaches to provide a way to wirelessly calibrate or synchronize thefrequencies of independent receivers of a precision geolocation or timereferencing system using two or more pulse transmissions from one ormore reference tags to define a time reference interval. As used herein,a pulse pair may be derived from a series of two or more pulses where anintertransmission (or interarrival) interval lies between any two pulseswithin the series of pulses. Instead of utilizing offset information ata processing end to adjust of time-of-arrival of object tag signals todetermine object location, the offset or representation thereof may befed back to the individual receivers to make internal adjustments inclock frequency to maintain uniformity or synchronization in internalclocks. UWB pulse transmissions enable good resolution due to theirshort duration but wideband transmissions may, in certain circumstance,also be acceptable. Where extreme position accuracy is not required,conventional lower frequency or narrower band RF pulse transmissions maybe utilized. The invention has applications beyond object location(e.g., navigation, remote timing, or other applications), thus thedisclosure a geopositioning is not intended to limit the invention tothat field. Unlike prior systems and methods, the actual location of thereference tag need not be known, nor is it required to tightly controlthe duration of the time reference interval transmitted by the referencetag. Using time differences of arrival, clock ratios may be computed foreach monitoring station relative to one monitoring station chosen asreference, i.e., master. These clock ratios are then used to adjust therelative clock frequency of each receiver to precisely match that of themaster. Finally, with all receivers adjusted for frequency lock, themethod and system of the U.S. Pat. No. 6,882,315 patent can be used todetermine the precise position of all (non-reference) tags within thearea of receiver coverage.

In addition to deploying the invention in an object location system,other applications include vehicle guidance and navigation which is alsoa form of object location. Vehicles include land, sea, air, and spacevehicles including motor vehicles, marine vessels, and aircraft wherepresent day expensive systems may be replaced with low-cost systemsprovided by the present invention. The invention may also be deployed inearth tunneling and boring machines to accurately guide the machinethrough the earth. Using ultra wideband in this application isparticular advantageous due to earth penetration advantages of UWBsignals. Satellite navigation system may also employ the methods andsystems herein for timing reference or correction.

Accordingly, the invention is not limited to the embodiments shown anddescribed herein but includes subject matter embraced by the appendedclaims.

What is claimed is:
 1. A method calibrating independent,spatially-located clocks of a geoposition system in order to geolocatean object having an associated object tag, said method comprising:utilizing respective frequencies of first and second spatially-locatedclocks to produce count values to effect measurement of an interarrivalinterval defined for an RF pulse pair received at each of a firstreceiver location and a second receiver location, determining, by aprocessor, a ratio of count values relative to the first and secondspatially-located clocks, and calibrating time indications of saidclocks based on the ration of count values.
 2. The method of claim 1,further comprising utilizing a ratio of an initial pulse pair tomaintain synchronization of said clocks during subsequent pulse pairtransmissions.
 3. The method of claim 2, further comprising wirelesslytransmitting said count values to a central processing hub thatdetermines object position according to said ratio.
 4. An objectlocation system, said system comprising: a reference transmitter totransmit at least two UWB pulses comprising short bursts of RF energy,multiple monitoring stations positionable at known locations, eachincluding a UWB receiver and a local clock that responds to receipt ofsaid UWB pulses to determine a clock count based on an interarrivalinterval, each said monitoring station further including transmitter totransmit a digital representation of said clock count to a centralprocessing hub, and said central processing hub including a processor tocompute a ratio of count values relative to first and second ones ofsaid multiple monitoring stations and to utilize said ratio to alignindications of timing references of said monitoring stations in order todetermine the position of said object.
 5. The object location system ofclaim 4, wherein said processor computes a second count value ratiobetween first and third ones of said monitoring stations and utilizessaid first and second count values to align respective local clocks todetermine the position of said object.
 6. The object location system ofclaim 5, wherein said processor utilizes a clock count value of one ofthe monitoring stations as a common reference in order to compute ratioswith respect to clock count values of other monitoring stations, andsaid processor determines object location by aligning local clocks ofother monitoring stations with said first monitoring station.
 7. Theobject location system of claim 4, wherein said reference transmittertransmit multiple pulse pair groups, and said processor correctsindications of clock drifts among said monitoring stations in order tomaintain synchronization of local clocks whereby to determine thelocation of object based upon subsequent object tag transmissions. 8.The object location system of claim 7, wherein said processor correctslocal clocks relative to higher order anomalies.
 9. The object locationsystem of claim 4, wherein said digital representation further includesa station identifier to identify respective ones of said monitoringstations.
 10. The object location system of claim 4, wherein saidprocessing hub is co-located with one of said monitoring stations. 11.The object location system of claim 4, wherein the monitoring stationsand processing hub are remotely located, and said monitoring stationstransmit said clock count to said central processing hub via anover-the-air RF transmission.
 12. The object location system of claim 4,further including a free-running cyclic ring counter that is indexed bysaid local clock of each said monitoring station in order to measurerespective clock count values during said interarrival interval of saidUWB pulses.